1. Field of the Invention
This invention relates to the field of extrusion die manufacture. In particular, the present invention pertains to the field of extrusion die manufacture by electrical discharge machining. More particularly still, the present invention relates to the manufacture of dies for use in forming extruded helical gears.
2. Description of the Prior Art
In the process of forming articles by extruding metal through a die that has the contour of the extrusion formed on its interior surface, it is well known that the extrusion expands after being forced through the forming surface or land of the die. This expansion is the result of elastic deformations that develop in the extrusion when it is forced through the die. The strain energy stored in the extrusion is released after it passes the forming surface of the die because the die generally has only a short forming land but a longer and larger space immediately beyond the land.
Unless the die provides a sufficiently large space beyond the forming orifice the extrusion will expand into tight contact with the interior surface of the die and produce excessively high frictional forces on this surface. The frictional forces may actually exceed the capacity of the forming press to force the extrusion through the die and could produce galling of the die surface and lead to premature failure. Furthermore, an excessive amount of energy is required to overcome the effect of the frictional forces that resist the extrusion forces applied by the press.
Consequently, extrusion dies typically furnish a greater annular space within the die beyond the forming surface than the space defined at the orifice of the die. Often electrical discharge machining methods are used to produce the forming surface of the die. According to this method, a carbon electrode or an electrode of copper-tungsten has its outer surface formed to the shape of the outer contour of the extrusion. The electrode is passed through a die blank and is energized with electrical energy so that arcing occurs between the electrode surface and the interior surface of the die blank. This arcing produces localized melting and chipping of the die blank surface as the electrode passes axially through the die thereby forming on the interior surface of the die blank the contour of the electrode. In order to furnish the larger annular space within the die that accommodates outward expansion of the extrusion a second tapered electrode may be subsequently passed partially through the die to form a cavity beyond the die orifice that is tapered outwardly thus providing the necessary increased volume.
If the extruded part has a noncircular cross section, particularly if it is lobed as is, for example, a gear wheel, it is most important that the portion of the die cavity beyond the forming orifice progressively increase in size along the length of the die in the circumferential sense. The prior art has recognized the difficulties associated with this expansion of the extrusion within the die. Extrusion dies typically are formed so as to produce a greater annular space beyond the forming surface than the space defined by the forming surface. Electrical discharge machining methods, wherein one carbon electrode having the desired shape of the part to be formed is passed through the die to establish the die land, are frequently employed. An electrical current produces a spark between the electrode and the inner surface of the die blank, which operates to remove metal from the die surface in the shape of the carbon electrode. In this way the forming surface of the die is established. However, in order to furnish the relief beyond the forming land, the prior art has typically used a second tapered electrode to form a cavity beyond the die land which is tapered outwardly to provide the requisite increased volume to accommodate the expansion.
However, because two distinct electrodes are used in conventional forming processes, it is extremely difficult to adapt the electrodes to eliminate geometric discontinuities particularly at the cross section where the tapered surface and the land forming surface intersect.
When the extruded part has a noncircular cross section, particularly when it is lobed, an additional difficulty is presented when the inner surface of the die is formed by a process that requires an electrode or forming tool to be inserted into the die for shaping purposes, but then to be withdrawn and the rest of the die surface formed either by another tool or by the same tool introduced into the die from the opposite direction. When this procedure is used, in addition to the difficulties associated with aligning the centerlines of the respective tools so that the axes are colinear, the tools must be additionally controlled so that at the cross section where the first shaping operation terminates and the second begins, the tools correspond circumferentially. This circumferential registry is required because of the nonuniform shape of the outer contour of the parts. For example, where the die is to be used for extruding gear teeth, the flanks of the teeth on the die must form a continuous and smooth surface along the full length of the die, even though the surface is shaped by two distinct passes of a tool through the die blank. When the gear teeth are helical, the difficulty of maintaining circumferential registry is compounded by the requirement that the forming tools rotate while they pass axially through the die blank.
Other and more conventional methods are known in the art for forming the interior surface of an extrusion die. For example, grinding and, before the die blank is hardened, milling and broaching techniques are available. Each of these, however, presents the same difficulties of circumferential misalignment and dimensional discontinuity as discussed previously with respect to electrical discharge machining. Grinding, however, increases the cost to produce the die.